浅谈关于ARC循环引用得问题-白红宇

浅谈关于ARC循环引用得问题

阅读量：7282 次

发布时间：2019-06-30

本文共 5343 字，大约阅读时间需要 17 分钟。

这段时间在研究关于ARC得循环引用导致变量不能释放，在此先介绍一本书英文书：

《Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch, and Blocks》

(《iOS与OS X多线程和内存管理》)

建议读英文原版，中文版看的我稀里糊涂的，后来被迫去看原版.这本书介绍了关于ARC的自动引用机制，这里的机制类似：当C语言的局部变量离开他的作用域之后就会被清除，这里的ARC也类似是这样的原理,如下面的代码一样,当离开｛｝之后obj会被nil。默认id是强指针

{/** You create an object and have ownership. */id __strong obj = [[NSObject alloc] init];/** The variable obj is qualified with __strong.* Which means, it has ownership of the object. */}/** Leaving the scope of variable obj, its strong reference disappears.* The object is released automatically.* Because no one has ownership, the object is disposed of.*/

不过有一种情况会导致在｛｝内的变量再离开作用域之后不会释放，这就是循环引用，下面我先贴一份没有循环引用的代码

//Test Class@interface Test : NSObject@property (nonatomic) id obj;- (void) setObj:(id)obj;@end@implementation Test- (void) setObj:(id)obj{    _obj = obj;}@end//Use Test Class- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    id test0 = [[Test alloc]init];    id test1 = [[Test alloc]init];    self.t0 = test0;    self.t1 = test1;}- (void) viewDidAppear:(BOOL)animated{    [super viewDidAppear:animated];    NSLog(@"test0 is %@",self.t0);    NSLog(@"test1 is %@",self.t1);    NSLog(@"test0 obj is %@",self.t0.obj);    NSLog(@"test1 obj is %@",self.t1.obj);    NSLog(@"viewDidApperar!");}/*  这里的输出结果：2014-11-18 11:39:31.654 circular[27065:60b] test0 is (null)2014-11-18 11:39:31.654 circular[27065:60b] test1 is (null)2014-11-18 11:39:31.655 circular[27065:60b] test0 obj is (null)2014-11-18 11:39:31.655 circular[27065:60b] test1 obj is (null)2014-11-18 11:39:31.655 circular[27065:60b] viewDidApperar!    */

* Leaving the scope of variable test0, its strong reference disappears.

* object A is released automatically. *

* Leaving the scope of variable test1, its strong reference disappears.

* object B is released automatically.

*

* At this moment, obj_ of object B has a strong reference to object A. *

* At this moment, obj_ of object A has a strong reference to object B. *

* memory leaked!! */

现在我贴一下存在循环引用的代码，这个时候局部变量的强指针相互引用，这里我们可以看见Test0的obj指向Test1，而Test1的obj指向Test0,导致释放不了.

//Test Class@interface Test : NSObject@property (nonatomic) id obj;- (void) setObj:(id)obj;@end@implementation Test- (void) setObj:(id)obj{    _obj = obj;}@end//Use Test Class- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    id test0 = [[Test alloc]init];    id test1 = [[Test alloc]init];    [test0 setObj:test1];    [test1 setObj:test0];        self.t0 = test0;    self.t1 = test1;}- (void) viewDidAppear:(BOOL)animated{    [super viewDidAppear:animated];    NSLog(@"test0 is %@",self.t0);    NSLog(@"test1 is %@",self.t1);    NSLog(@"test0 obj is %@",self.t0.obj);    NSLog(@"test1 obj is %@",self.t1.obj);    NSLog(@"viewDidApperar!");}/*这里的输出结果：2014-11-18 11:42:35.555 circular[27654:60b] test0 is 
     
      2014-11-18 11:42:35.556 circular[27654:60b] test1 is 
      
       2014-11-18 11:42:35.556 circular[27654:60b] test0 obj is 
       
        2014-11-18 11:42:35.556 circular[27654:60b] test1 obj is 
        
         2014-11-18 11:42:35.558 circular[27654:60b] viewDidApperar!*/

一般的解决办法就是weak obj,代码如下：

//Test Class@interface Test : NSObject@property (nonatomic,weak) id obj;- (void) setObj:(id)obj;@end@implementation Test- (void) setObj:(id)obj{    _obj = obj;}@end//Use Test Class- (void)viewDidLoad{    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    id test0 = [[Test alloc]init];    id test1 = [[Test alloc]init];    [test0 setObj:test1];    [test1 setObj:test0];        self.t0 = test0;    self.t1 = test1;}- (void) viewDidAppear:(BOOL)animated{    [super viewDidAppear:animated];    NSLog(@"test0 is %@",self.t0);    NSLog(@"test1 is %@",self.t1);    NSLog(@"test0 obj is %@",self.t0.obj);    NSLog(@"test1 obj is %@",self.t1.obj);    NSLog(@"viewDidApperar!");}/*这里的输出结果：2014-11-18 11:46:19.803 circular[28360:60b] test0 is (null)2014-11-18 11:46:19.804 circular[28360:60b] test1 is (null)2014-11-18 11:46:19.804 circular[28360:60b] test0 obj is (null)2014-11-18 11:46:19.805 circular[28360:60b] test1 obj is (null)2014-11-18 11:46:19.805 circular[28360:60b] viewDidApperar!*/

那么我们现在来了解一下为什么使用weak之后就会解除循环引用呢？

当我们直接写 id __weak obj = [[NSObject alloc] init];

会提示警告：因为使用了weak，所以obj并不持有该对象，那么[[NSObject alloc] init]被创建之后没有人持有它，那么它就会被释放，所以这里应该修改成，当执行｛｝之后 obj0 和 obj1 都会被释放，因为当obj0被释放之后，obj1会变成nil因为weak引用不持有对象，当引用对象的强指针消失时，weak声明的指针也会自动null

{/** You create an object and have ownership. */id __strong obj0 = [[NSObject alloc] init];/** The variable obj0 is qualified with __strong.* Which means, it has ownership of the object. */id __weak obj1 = obj0;/** variable obj1 has a weak reference of the created object */} /** Leaving the scope of variable obj0, its strong reference disappears.* The object is released automatically.* Because no one has ownership, the object is discarded. */

那么下面这个例子就说明了weak会自动null

id __weak obj1; //__strong ,__weak 声明的变量会自动设置成为nil，这里等同于 id __weak obj1 = nil;

{id __strong obj0 = [[NSObject alloc] init];obj1 = obj0;NSLog(@"A: %@", obj1); }NSLog(@"B: %@", obj1);The result is:A: 
           
             B: (null)

转载于:https://www.cnblogs.com/zuopeng/p/4105356.html

你可能感兴趣的文章